The invention relates generally to Asymmetrical Digital Subscriber Line (ADSL) systems and more specifically to a method and an arrangement for securing an ADSL connection via a telephone line between an ADSL transceiver in a central office and an ADSL transceiver connected together with a telephone to the telephone line on a subscriber""s premises.
This invention is related to the invention described in Swedish patent application No. 9801646-2.
As described in the above patent application, in ADSL systems, data signals are transferred at the same time as telephony on a common telephone line.
The telephone traffic and the ADSL traffic have to be able to coexist on the telephone line, i.e. one should be able to talk in a telephone that is connected to the telephone line at the same time as a computer on the subscriber""s premises is connected to the telephone line without disturbing either traffic.
In traditional ADSL technology, this is solved by means of a so called splitter, which in principle is a high-order filter which is connected in series with the telephone and makes the telephone xe2x80x9cinvisiblexe2x80x9d for ADSL signals. By means of such a filter, disturbances from the telephone to an ADSL modem are avoided, e.g. when the telephone goes off-hook, i.e. when the impedance in the telephone is abruptly changed from a high to a low value. Without such filter, this would result in a changed attenuation of the ADSL signals and, consequently, in a changed signal-to-noise ratio. In its turn, this could cause the ADSL system to loose synchronization, since the ADSL modem is set up for signal-to-noise ratios that existed when the modem was initialized.
The major problem with such a splitter filter is that it is very expensive and bulky. Moreover, it takes a specialist to install such a filter, which makes it even more expensive.
To facilitate the exploitation of ADSL, a form of ADSL has been defined, for which it will be possible to run telephone traffic and ADSL traffic simultaneously without such splitters but with reduced performance. This application is popularly called ADSL-Lite or splitterless ADSL.
However, the risk of loosing the synchronization still remains in those systems.
Upon a call to the telephone, the telephone station first sends out a ringing signal having a frequency between 20 and 60 Hz depending on the market, and having a signal level which is specified to be at least 40 V r.m.s. at the telephone. Often, the level is considerably higher.
When the telephone goes off-hook, the impedance is abruptly changed to a considerably lower level.
In the case of ADSL, it is of particular interest which impedance the telephone has in the ADSL band.
For ADSL-Lite, a downstream band, i.e. traffic from the station to the subscriber, of around 550 kHz is considered, while an upstream band, i.e. traffic from the subscriber to the station, of 138 kHz is considered.
Measurements of some typical telephones give the following impedance values:
On-hook impedance @ 100 kHz=3 kxcexa9
On-hook impedance @ 400 kHz=800xcexa9
Off-hook impedance @ 100 kHz=60xcexa9
Off-hook impedance @ 400 kHz=8xcexa9
The driving and terminating impedance of the ADSL modem is normally adapted to the characteristic impedance of the telephone line within the ADSL band, which is around 100xcexa9. Considering the load that the modem on the subscriber side sees looking out onto the line, it will be very different in the above cases.
Suppose that the characteristic impedance of the line is equal to 100xcexa9 and that the line is correctly terminated on the station side with 100xcexa9 in the ADSL band.
The load seen looking out from the modem is of importance only for the upstream band, i.e. up to 138 kHz:
Load On-hook @ 100 kHz=97xcexa9
Load Off-hook @ 100 kHz=37xcexa9
In the same manner as the load changes, the terminating impedance for downstream traffic will change:
Terminating Impedance On-hook @ 400 kHz=89xcexa9
Terminating Impedance Off-hook @ 400 kHz=7.5xcexa9
In principle, this means that both the incoming and outgoing signals will be considerably more attenuated for the off-hook case than with the telephone in on-hook condition. This implies a great risk for the ADSL system to loose the synchronization, since bit allocation is carried out in correspondence to the conditions at hand during the training sequence which is normally run with the telephone in on-hook condition.
ADSL utilizes Discrete Multi-Tone (DMT) coding according to which the available bandwidth is divided into 4.3 kHz channels. During the training sequence, the signal-to-noise ratio in all channels is measured and, thereafter, it is decided how many data bits can be allocated to a certain carrier channel.
For ADSL-Lite, 256 QAM (Quadrature Amplitude Modulation) is used, which means that a maximum of 8 bits can be allocated to each carrier channel. When the telephone goes off-hook, the signal levels will be lowered due to further attenuation, which means that the signal-to-noise ratio will be degraded.
The upstream traffic will have a three times lower signal level or a signal-to-noise ratio degradation of 9.5 dB, while the degradation downstream can be as much as twelve times or 21.5 dB. Due to this considerable impedance change in the ADSL band in off-hook condition of the telephone in an ADSL-Lite application, the signal-to-noise ratio is degraded so much that the system will loose the synchronization and have to be restarted by means of a new training sequence.
The object of the invention is to secure, at the central office end, an ADSL connection between an ADSL transceiver in a central office and an ADSL transceiver at a telephone via a telephone line that is used in common with the telephone, in connection with a call to the telephone.
This is attained in accordance with the invention by detecting a ringing request directed to the telephone and, in response thereto, lowering the bit rate allocated to the ADSL connection until the ringing request either has disappeared without the telephone having gone off-hook or until the telephone goes on-hook again after having been off-hook.
Hereby, the risk of loosing the synchronization will be more or less eliminated.